Constant velocity ratio universal joint of the tripode type

ABSTRACT

A constant velocity ratio universal joint of the tripode type includes an outer joint member with three tracks extending lengthwise of the joint member, and an inner joint member having three arms extending outwardly into the tracks of the outer joint member. Each arm carries a respective roller having an external surface which engages opposed side portions of the associated track. Each roller is able to tilt within its associated track and move therealong so that the inner and outer joint members are able to move axially relative to one another and undergo relative articulation. In the invention 1) the tracks in the outer joint member, in respect of their alignment relative to a datum comprising the axis of rotation of the outer joint member, and 2) the arms of the inner joint member, in respect of their alignment relative to a datum comprising a plane perpendicular to the axis of rotation of the inner joint member, at least the tracks in the outer joint member are so inclined relative to the respective datum as substantially to eliminate the third order transmission error from the joint.

DESCRIPTION OF INVENTION

This invention relates to constant velocity ratio (homokinetic)universal joints of the tripode type, such a joint comprising an outerjoint member having a rotational axis and three tracks equallycircumferentially spaced about its rotational axis and extendinglengthwise of the joint member; and an inner joint member having arotational axis and three arms equally circumferentially spaced aboutits rotational axis and extending outwardly from the inner joint memberinto the tracks of the outer joint member, each arm carrying arespective roller having an external surface which engages opposed sideportions of the associated track, each roller being able to tilt withinits associated track and move therealong so that the inner and outerjoint members are able to move axially relative to one another andundergo relative articulation. Such a joint will hereafter be referredto as a joint of the kind specified.

Tripode-type joints of the kind specified are widely used in motorvehicle drive lines, for example as the inboard universal joints ofdrive shafts extending laterally to drivable wheels of a vehicle. Whenused in motor vehicles, particularly passenger cars, the transmissioncharacteristics of universal joints in terms of not causing anyvibrations in the vehicle drive line are of great importance, to reducenoise, vibration and harshness detectable by persons travelling in thevehicle.

In a conventional tripode joint of the kind specified, the tracks in theouter joint member extend parallel to the rotational axis of such jointmember. The arms of the inner joint member are perpendicular to therotational axis of the inner joint member. There have been proposals fortripode joints in which the tracks and arms are at other orientations,for example U.S. Pat. No. 3,990,267, shows a tripode joint wherein thetracks of one joint member are skewed relative to the rotational axisthereof, for the purpose of generating an axial force when the jointtransmits torque. It has been proposed, in JP Utility Model publication63-115927, that the arms of the inner joint member should be inclined atan angle other than 90° to the rotational axis of the joint member, butthis does not eliminate the transmission error described hereafter.

Although commonly referred to as a constant velocity ratio (homokinetic)universal joint, the conventional tripode joint of the kind specifiedonly behaves as a truly constant velocity ratio joint if it is installedin such a way that rotation of the joint when articulated does not causea change in the inclination of the rotational axis of the inner jointmember relative to the axis of the outer joint member. If not installedthus, and one joint member is rotated at a constant angular velocity,when the joint is articulated, the angular velocity of the other jointmember will cyclically decrease and increase, although the mean angularvelocity of both joint members is, of course, the same. This deviationof the joint from true constant velocity characteristics, referred toherein as transmission error, is predominantly third order, being asinusoidal variation with a frequency of three times the rate ofrotation of the joint. There are other orders of transmission error, butFourier analysis shows that, for a geometrically perfect joint having noother geometric distortions, the third order component represents 99.8%of the RMS value of all transmission errors. The transmission errorarises from the orbital motion which the centre of the inner jointmember is caused to undergo if the rotational axis of the outer jointmember is held fixed, and if the opposite end of a shaft connected tothe inner joint member is constrained to rotate about a fixed point theinclination of such shaft must change. If such shaft were(theoretically) infinitely long the change in inclination would be zero,or a change in shaft inclination can be avoided if a tripode-type jointis used at the other end of the shaft, providing the arms of the innerjoint members of the two joints are in phased relationship with oneanother and the articulation angles of the two joints are equal.Although the shaft connecting the two joints is still subject to anorbiting motion about its nominal axis it remains parallel to such axisand its angular velocity does not fluctuate.

The transmission error above described is believed to be a significantfactor in the overall characteristics of a tripode-type joint, in termsof excitation of driveline vibrations, and accordingly it is an objectof the present invention to reduce such error and the disadvantageousconsequences thereof. The absence of speed fluctuations would also bebeneficial in specialised applications such as instrument drives orprecision mechanisms or in high rotary inertia drives.

SUMMARY OF THE INVENTION

According to the present invention, we provide a constant velocity ratiouniversal joint of the tripode type, comprising an outer joint memberhaving a rotational axis and three tracks equally circumferentiallyspaced about its rotational axis and extending lengthwise of the jointmember; and an inner joint member having a rotational axis and threearms equally circumferentially spaced about its rotational axis andextending outwardly from the inner joint member into the tracks of theouter joint member, each arm carrying a respective roller having anexternal surface which engages opposed side portions of the associatedtrack, each roller being able to tilt within its associated track andmove therealong so that the inner and outer joint members are able tomove axially relative to one another and undergo relative articulation,wherein, of

1. the tracks in the outer joint member, in respect of their alignmentrelative to a datum comprising the axis of rotation of the outer jointmember, and

2. the arms of the inner joint member, in respect of their alignmentrelative to a datum comprising a plane perpendicular to the axis ofrotation of the inner joint member,

at least the tracks in the outer joint member are so inclined relativeto the respective datum as substantially to eliminate the third ordertransmission error from the joint.

As mentioned above, in a conventional tripode-type universal joint ofthe kind specified, the tracks in the outer joint member extend parallelto the rotational axis of such joint member. The arms of the inner jointmember are perpendicular to the rotational axis of the inner jointmember. It has been found that the third order transmission error, in ajoint according to the invention, can be eliminated by providing thatthe tracks in the outer joint member are at an orientation other thanparallel to the rotational axis of the joint member, or by providingthat the tracks in the outer joint member are at an orientation otherthan parallel to the rotational axis of the joint member and the arms ofthe inner joint member are at an orientation other than perpendicular tothe rotational axis of the joint member.

The usual configuration of the inner and outer joint members of atripode joint of the kind to which the present invention relates is thatthe outer joint member is generally in the form of a hollow componentclosed at one end, such end being provided with a stub shaft, a driveflange or other means whereby torque may be transmitted to the outerjoint member. The inner joint member is adapted for connection to ashaft element which extends from the joint at its end opposite theclosed end of the outer joint member.

In one embodiment of joint according to the invention, the arms of theinner joint member may be arranged as in a conventional joint, i.e.perpendicular to the rotational axis of the inner joint member, whilethe tracks in the outer joint member are inclined so as to diverge fromone another as they extend from the closed end of the outer joint memberto the open end thereof, the centre lines of the tracks lying inrespective planes which also contain the rotational axis of the outerjoint member, and being inclined at an angle of 3° to 4° to such axis.

For a typical joint, the optimum angle has been found to be about 3.8°.For a theoretical complete elimination of the third order transmissionerror, the angle of inclination of each track relative to the rotationalaxis of the outer joint member may be determined by the expression0.9998 arctan (r/L) where r is the pitch circle radius of the tracks atthe normal axial position of the inner joint member within the outerjoint member, and L is the length of the shaft connected to the innerjoint member.

In an alternative embodiment of joint according to the invention, thetracks in the outer joint member can be inclined relative to therotational axis of the outer joint member in the opposite sense to thatreferred to above, i.e. the tracks can converge as they extend from theclosed end of the outer joint member to the open end from which a shaftelement connected to inner joint member extends. At the same time, thearms of the inner joint member are inclined relative to the conventionalorientation perpendicular to the rotational axis of the inner jointmember, such inclination being in the same sense as the inclination ofthe tracks relative to the rotational axis of the outer joint member,i.e. the arms being inclined towards the opposite end of a shaft elementconnected to the inner joint member.

In this case, the inclination of each track relative to the rotationalaxis of the outer joint member, and the inclination of each arm relativeto the perpendicular to the rotational axis of the inner joint member,may be in the range 3° to 4°, preferably approximately 3.8°.

The expression for the inclination of the tracks and arms is given bythe expression 0.9949 arctan (r/L), r and L being as defined above.

If the tracks in the outer joint member are inclined, it will beappreciated that the pitch circle radius of the tracks varies along thelength of the outer joint member. In accordance with either of theexpressions above set forth for calculating track inclination, it willbe understood that theoretically the configuration of the tracks wouldbe a shallow curve, the inclination of the tracks varying along theirlength. The theoretical form of the tracks is given by the approximation

    r≈r.sub.o.e.sup.(x/L)

where

r=pitch circle radius of track

r_(o) =pitch circle radius at axial datum position

x=axial displacement from the datum position

L=length of shaft connected to inner joint member

Such curved tracks, of varying inclination along their length aredifficult to manufacture, and in practice substantial elimination of thethird order transmission error can be achieved if the tracks arestraight, their inclination being determined by the pitch circle radiusat the normal operating position of the inner joint member axially ofthe outer joint member.

The surprising aspect of the findings according to the invention is thatthey reduce the transmission error not only at one angle of jointarticulation but over the whole range of angles within which such jointsare required to articulate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, of which:

FIG. 1 is a diagrammatic section through a conventional tripode joint,and the accompanying graph shows the transmission error associatedtherewith;

FIG. 2 is a view as FIG. 1, showing a first embodiment of jointaccording to the present invention:

FIG. 3 is a further view showing another embodiment of joint accordingto the present invention; and

FIG. 4 is a view showing the dimensions applicable to the calculation oftrack and arm inclination in joints according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to FIG. 1 of the drawings, the prior art tripode typeof universal joint there illustrated comprises an outer joint memberindicated generally at 10 and an inner joint member indicated generallyat 11. The outer joint member is generally in the form of a hollowcomponent, closed at one end 12 from which end a stub shaft 13 extendsfor torque transmitting connection of the outer joint member to anothercomponent in, for example, a vehicle driveline. The rotational axis ofthe outer joint member is indicated at 14. From the closed end 12 of theouter joint member there extend three tracks 15, equallycircumferentially spaced about the axis of rotation 14 of the jointmember. Each track comprises opposed surfaces which are generally in theform of parts of a cylindrical or substantially cylindrical surface. Thecentre lines of the tracks, as indicated at 16, and which are thecentral longitudinal axes of the cylinders of which the tracks surfacesform part, are parallel to the rotational axis 14 of the outer jointmember. In the illustrated joint the tracks 15 are open towards theexternal circumference of the outer joint member; in some designs oftripode joint the tracks are not thus open and the outer joint member isin the form of a cup with an unbroken external circumferential surface.

The inner joint member 11 is an annular component, having torquetransmitting engagement with a shaft element 17 extending therefrom outof the open end of the outer joint member 10, the rotational axis of theinner joint member and shaft 17 being indicated at 18. Three arms, oneof which is shown at 19, extend outwardly from the inner joint memberinto the tracks 15 in the outer joint member, such arms havingrespective axes as indicated at 20 for the arm 19, which axes lie in aplane perpendicular to the rotational axis 18 of the inner joint member.Each arm 19 carries a respective roller as seen at 21, which is able torotate about the arm and move lengthwise of the arm, i.e. axially withrespect to the arm axis 20. Each roller 21 has a part-spherical externalsurface, which enables the roller to tilt within the track 15 in whichit engages. This condition is illustrated in FIG. 1, in which the jointis in the articulated condition with the rotational axes 14, 18 of theouter and inner joint members inclined to one another.

As hereinbefore described, a transmission error is associated with aconventional tripode joint as above described. If one joint member isrotated at a constant angular velocity when the joint is articulated,the angular velocity of the other joint member will cyclically decreaseand increase. This transmission error, which is predominantly a thirdorder error, is a sinusoidal variation with a frequency of three timesthe rate of rotation of the joint, as illustrated in the diagram to theright of the joint shown in FIG. 1.

FIG. 2 shows a first embodiment of universal joint according to thepresent invention. The basic structure of the joint, comprising an outerjoint member and inner joint member and associated parts is the same asthat described above and will not therefore be described again. Thejoint differs from the joint of FIG. 1, however, in that the centre lineof each track of the outer joint member is inclined to the rotationalaxis of the outer joint member instead of being parallel thereto. InFIG. 2, the centre line of the illustrated track is indicated at 16a,and this centre line is inclined at an angle X to a line 14a parallel tothe rotational axis 14 of the outer joint member. The centre line 16alies in a plane which also contains the axis 14, and the inclination ofeach track is such that the tracks diverge from one another as theyextend from the closed end to the open end of the outer joint member.

By appropriate selection of the angle X, the third order transmissionerror in the joint can be reduced to zero as shown diagrammatically inFIG. 2. For the type of joint typically used in vehicle drivelines, theangle X is between 3° and 4°, preferably approximately 3.8°.

FIG. 3 shows a further embodiment of joint according to the invention,which again is the same in respect of its general structure as the jointof FIG. 1. In this embodiment, both the centre lines of the tracks inthe outer joint member, and the arms of the inner joint member areinclined relative to, respectively, the rotational axis of the outerjoint member and the plane perpendicular to the rotational axis of theinner joint member.

FIG. 3 shows one of the tracks in the outer joint member arranged withits centre line 16b inclined at an angle Y to a line 14b parallel to therotational axis 14 of the outer joint member. The tracks in theembodiment of FIG. 3 are inclined in the opposite sense to theinclination thereof in the embodiment of FIG. 2, i.e. in FIG. 3 thetracks converge as they extend from the closed end of the outer jointmember to the open end thereof.

In FIG. 3, the axis 20b of the illustrated arm 19 of the inner jointmember is inclined at an angle Z to the plane perpendicular to the axisof rotation 18 of the inner joint member. The arms of the inner jointmember are inclined in the same sense as the tracks in the outer jointmember, i.e. the arms are melted towards the end of the shaft element 17remote from that which is connected to the inner joint member. The axesof the arms intersect the axis of the inner joint member.

The angle Y is substantially equal to the angle Z, and this angle alsois preferably, for typical joints, in the range 3° to 4°, preferably3.8°.

FIG. 4 of the drawings shows how the angle X in the embodiment of FIG. 2or the angles Y and Z in the embodiment of FIG. 3 may be calculated.Exact values of these angles depend on the pitch circle radius of thetracks in the outer joint member, i.e. the distance from the rotationalaxis of the outer joint member to the centre lines of the tracks, andthe overall length of the shaft element connected to the inner jointmember. In each case, the angle X, Y, or Z is approximately equal toarctan (r/L). More precisely, the angle X in the embodiment of FIG. 2equals 0.9998 arctan (r/L) whilst the angles Y and Z in the embodimentof FIG. 3 each equals 0.9949 arctan (r/L).

It will be appreciated that since, if the tracks are inclined, r varies,the angle of inclination of the tracks will theoretically vary along thelength of the tracks. Theoretically the configuration of the tracks is ashallow curve so that the pitch circle radius of the track varies inaccordance with the expression r≈r_(o).e.sup.(x/L). In practice,however, substantial elimination of the third order transmission errorcan be achieved if the tracks are straight, their inclination beingdetermined in accordance with the pitch circle radius thereof at thenormal operating position of the inner joint member axially of the outerjoint member. In typical use of a joint in a motor vehicle drive line,most of the axial displacement of the inner joint member from its normaloperating position is of small magnitude, with displacements of greatermagnitude only rarely arising.

The features disclosed in the foregoing description or the followingclaims or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately or in any combination of such features, be utilised forrealising the invention in diverse forms thereof.

We claim:
 1. A constant velocity ratio universal joint of the tripodetype, comprising:an outer joint member having a rotational axis andthree tracks, each track having opposed side portions and a center lineand said center lines of the tracks being equally circumferentiallyspaced about said rotational axis of the outer joint member andextending lengthwise of the outer joint member in respective planeswhich also contain said rotational axis of the outer joint member; apoint in each said track center line being disposed at a pitch circleradius r relative to said rotational axis of the outer joint member;said outer joint member having a closed end and an open end; an innerjoint member having a rotational axis and three arms equallycircumferentially spaced about said rotational axis of the inner jointmember and extending perpendicularly to said rotational axis of theinner joint member outwardly of the inner joint member into respectivesaid tracks of the outer joint member; a respective roller carried byeach arm and having an external surface which engages opposed sideportions of the respective said track, each roller being able to tiltwithin its respective track and move there along so that the inner andouter joint members are able to move axially relative to one another andundergo relative articulation; a shaft connected to the inner jointmember and extending therefrom along said rotational axis thereof out ofsaid open end of the outer joint member, said shaft having a length L;wherein the center lines of the tracks are inclined to said rotationalaxis of the outer joint member so as to diverge therefrom as they extendfrom said closed end of the outer joint member to said open end thereof,at an angle approximately equal to arctan (r/L), whereby third ordertransmission error is substantially eliminated form the joint.
 2. Ajoint according to claim 1, wherein said inclination of at least thetracks is in the range 3° to 4°.
 3. A joint according to claim 2,wherein said inclination is approximately 3.8°.
 4. A joint according toclaim 1, wherein said angle equals 0.9998 arctan (r/L).
 5. A constantvelocity ratio universal joint of the tripode type, comprising:an outerjoint member having a rotational axis and three tracks, each trackhaving opposed side portions and a center line and said center lines ofthe tracks being equally circumferentially spaced about said rotationalaxis of the outer joint member and extending lengthwise of the outerjoint member in respective planes which also contain said rotationalaxis of the outer joint member; a point in each said track center linebeing disposed at a pitch circle radius r relative to said rotationalaxis of the outer joint member; said outer joint member having a closedend and an open end; an inner joint member having a rotational axis andthree arms equally circumferentially spaced about said rotational axisof the inner joint member and extending outwardly of the inner jointmember into respective said tracks of the outer joint member; arespective roller carried by each arm and having an external surfacewhich engages opposed side portions of the respective said track, eachroller being able to tilt within its respective track and move therealong so that the inner and outer joint members are able to move axiallyrelative to one another and undergo relative articulation; a shaftconnected to the inner joint member and extending therefrom along saidrotational axis thereof out of said open end of the outer joint member,said shaft having a length L; the center lines of the tracks in theouter joint member being inclined to said rotational axis of the outerjoint member so as to converge towards one another as they extend fromthe closed end of the outer joint member to the open end thereof; thearms of the inner joint member being inclined relative to a planeperpendicular to the rotational axis of the inner joint member, suchinclination being towards said open end of the outer joint member; eachsaid inclination of the arms and the tracks being at an angleapproximately equal to arctan (r/L), whereby third order transmissionerror is substantially eliminated from the joint.
 6. A joint accordingto claim 5, wherein said inclination of at least the tracks is in therange 3° to 4°.
 7. A joint according to claim 6, wherein saidinclination is approximately 3.8°.
 8. A joint according to claim 5,wherein said angle equals 0.9949 arctan (r/L).